Method for producing microhole structures

ABSTRACT

The invention relates to a novel method for producing microhole structures. According to said method, the material used to produce said microhole structures is applied to a substrate surface provided with a relief structure, by means of an angular coating process. In order to achieve the desired pattern of holes, the relief structure has a continuous network of first surface elements and second surface elements located thereinbetween, the local surface normal vectors of the first surface elements forming a small angle with the unit vector, and the local surface normal vectors of the second surface elements forming a small angle with the direction vector of the coating.

The invention relates to a method for manufacturing microholestructures.

Microhole structures for the fine filtration for example of fluids andfor the filtering of radiation or for the shielding of radiation havebeen known for a long time. Here, it is usually the case of regular holestructures with webs between the holes, which are connected in alarge-surfaced manner. For the filtering of radiation or shielding ofradiation, the webs are mostly metallic and have a high conductivity.Metallic and non-metallic materials are suitable for fine filtration.Since microhole structures with very thin webs when unsupported have astability which is too low for the filtration, these are supported by asecond grid which is considerably larger with respect to the dimensionsof the hole and web. For filtering the radiation, the hole structuresmay be located on a substrate which is optically transparent in thewavelength region which is of interest with regard to the filtering. Theholes of these microhole structures may be almost round or may also beelongate in one direction. Typical dimensions of the holes lie in theregion of 0.1 μm to 100 μm. In particular, the manufacture of microholestructures with typical hole dimensions in at least one direction of 0.1μm to 100 μm has been very expensive up to now since very expensivestructuring methods such as the LIGA method or photolithography andinterference lithography have been needed to be used for this, incombination with etching or lift-off techniques.

Hole structures with minimal hole dimensions of >approx. 1 μm may beformed in the laboratory lithographically by way of contact exposure.With this, firstly a mask is created by way of electron-beam writing.This, for duplication, is pressed against a substrate coated with aphotoresist, e.g. a thin film onto glass or silicon. During the exposureprocedure only the regions of the photoresist which are not covered bythe mask are irradiated with UV-radiation. In the exposed regions thephotoresist has a significantly different solubility rate in thesubsequent development process in comparison to the unexposed regions.With positive resists, the exposed regions dissolve quicker, withnegative resists the unexposed ones. Due to this, after developing, asurface relief arises, which given a suitable selection of the exposureand developing parameters, masks the very thin film at the location ofthe webs and leaves them free at the location of the holes. The film maysubsequently be etched in a wet-chemical manner or by way ofion-etching, and the photoresist may be removed.

Another technology is the lift-off method. With this, firstly thesubstrate is coated with photoresist and this is structurised.Subsequently the substrate including the photoresist structure isprovided with the thin film by way of a vacuum method such as vapourdeposition or sputtering. By way of dissolving the photoresiststructure, the film is lifted away at these locations. The mask foretched structures and for structures according to the lift-off method,given an otherwise equal processing, need to be complementary.

Photolithography in combination with electroshaping is also counted asbelonging to the known structuring methods, which is particularlyapplied to thick layers which are to be structured. This method is alsoindicated as a low-cost LIGA method.

The contact exposure method has the disadvantage that it may not be usedindustrially for hole dimensions <1 μm, since the reject rate wouldbecome too high due to the unavoidable variation of the distance betweenthe mask and the substrate.

The exposure of the photoresist may also be effected with projectionexposure methods. With this, the mask is typically projected onto thephotoresist layer, reduced in size in a ratio of 5:1. The wholesubstrate is exposed in a step-and-repeat process by way of repeatedexposure of the same pattern on the mask. The projection exposure hasthe advantage that with this method one may also industriallymanufacture structures <1 μm in the photoresist layer. However, aprojection exposure machine with exposure wavelengths in the low UVregion are required. Such projection exposure machines have highinvestment costs. Furthermore for the projection exposure, on account ofthe low depth of field of the imaging, one further requires extremelyplane substrates which as a rule may only be obtained by way ofexpensive surface treatment processes such a lapping and polishing. Thismeans that the costs for the substrate to be applied increase.

Interference lithography is particularly suitable for the formation ofperiodic structures (grid structures) which has already been suggestedfor the manufacture of microhole structures. With this technique, thephotoresist with the interference pattern is exposed to at least two ormore coherent wave fields which superimpose. The period A of the grid,given a symmetrical incidence of the two waves, is given by thefollowing relation:Λ=λ₀/2sinθ_(i)with λ₀ equal to the wavelength of the coherent wave fields and θ_(i)equal to the angle which the propagation directions of the incidentwaves encloses with the normal to the exposed surface.

With this, line grids are produced on exposure. The manufacture ofcrossed grids and hexagonal grids by way of two subsequent exposureswith the intermediate rotation of the substrate by 90° and 60°respectively is also known. After developing the photoresist, eitherfree-standing photoresist columns, or a continuous surface reliefarises.

The coating of linear surface relief grids under oblique incidence isknown for manufacture of polarisers for the near infra-red. Due to thecasting of the shadow, only one flank of the line grid is coated. Forthis, one mostly uses vapour deposition methods. It is however alsopossible to apply specially optimised sputter techniques. In the case ofthe polariser, with oblique coatings with a metal, one may then speak ofself-adjusted metallic strip conductors. A transfer of this technologyto the manufacture of microhole structures is however not obvious sincethe oblique coating of crossed grids or hexagonal grids would lead togreat demands with regard to adjustment (trimming). The propagationdirection of the coating cluster varies over the surface of thesubstrate. The casting of the shadow in a first approximation may beobserved as the casting of a shadow of a point light source. Sincehowever the distance between the source and the substrate in a vacuumapparatus may not be selected infinitely large, a local change of thepropagation direction may not be avoided. Thus only surface reliefswhich, as in the case of a line grid, are tolerant with respect to achange of the propagation direction of the coating cluster, are suitableas self-adjusting masks for oblique coating.

It is the object of the present invention to specify a method formanufacture of microhole structures which is inexpensive and permitsminimal hole dimensions of up to 0.1 μm.

According to the invention, this object is achieved by a method with thefeatures of claim 1. Advantageous further embodiments of the methodaccording to the invention are to be deduced from the dependent claims.

By way of the formation of a substrate with a relief structure on asurface and an oblique coating of the relief structure with the materialof the microhole structure, the substrate itself is used as a mask sothat inasmuch as this is concerned, one requires no adjustment(trimming) (“self-adjustment”), and the inaccuracies which this entailsare avoided. By way of this it is rendered possible to obtain holedimensions of down to 0.1 μm in an industrial manner.

For achieving the desired formation as a microhole structure, it isnecessary for the relief structure to have a continuous network of firstsurface regions whose local surface normal vectors enclose a small anglewith the unit vector of the surface, and of second surface regionsbetween the first surface regions, whose local surface normal vectorsenclose a small angle with the direction vector of the coating.

A particular economical efficiency of the method results if the reliefstructure of the substrate is formed by replication, of an originalstructure. At the same time, the original structure is preferably formedby a photolithographic method, in particular by way of interferencelithography, and an embossing punch of this is manufactured by way ofgalvanic deformation. By way of a subsequent process such as embossingor casting, the relief structure may be copied onto a multitude ofsubstrates. Preferred materials into which the relief may be replicatedare plastics, sol-gel layers and glass.

The invention is hereinafter described by way of the embodiment examplesrepresented in the Figures. There are shown in:

FIG. 1 the formation of a relief structure with truncated cone shapedprojections on a substrate surface,

FIG. 2 the acting manner of the oblique coating with the reliefstructure according to FIG. 1,

FIG. 3 the acting manner of the oblique coating with a relief structurewith truncated cone shaped recesses,

FIG. 4 a metallic microhole structure for filtering infrared radiation,and

FIG. 5 the course of the process with the manufacture of a microholestructure for the fine filtration of fluids.

The oblique coating has a preferred direction of coating. This, in thecase of known oblique coating of linear structures, is selectedperpendicular to the direction of the translation invariancy. For theoblique coating for the production of hole structures there is the newrequirement that the casting of the shadow of the raised region of thesurface structure does not lead to an interruption of the web structuresurrounding the hole. This may be ensured by various embodiments of thesurface relief:

-   A) an arrangement of parallelepiped, cylinder-shaped and truncated    cone shaped as well as similar projections on a plane or    approximately plane surface. With this arrangement the structure    heights and the direction of incidence of the coating material need    to be matched to one another in a very accurate manner, i.e. this    arrangement is sensitive to adjustment. Changes in the coating    direction lead very quickly to a change in the hole shape.

FIG. 1 shows a perspective representation of such an arrangement withwhich the projections are truncated cone shaped. FIG. 2 represents theoblique coating of this arrangement, from which the coated regions 1 andthe non-coated regions 2 lying in the shadow of the projections may beseen.

-   B) The negative of the surface relief from A): recesses in a plane    or approximately plane surface. This arrangement is considerably    more advantageous than arrangement A). However, regions of the wall    of the recess are also coated. This relief structure may be obtained    by way of replication of an original structure in the design of the    arrangement A). FIG. 3 schematically shows the oblique coating of    the arrangement B).-   C) A continuous surface relief which, observed in one direction    (x-direction), is modulated alternately to a greater and then to a    lesser extent and in the direction perpendicular to this    (y-direction) comprises webs at shorter distances. With the oblique    coating in the x-direction, the weakly modulated regions are fully    coated. The webs in the y-direction on account of the casting of the    shadow produce the hole structures with the oblique coating. The    more elongate the hole structures, the less sensitive is the    structure with respect to adjustment (trimming) errors with the    oblique coating.

FIG. 4 in a plan view shows a metallic microhole structure for filteringinfrared radiation, which has been obtained by way of oblique coating ofsuch a relief structure.

For the three mentioned designs A), B) and C) of the surface relief, thetwo following conditions are valid, with the assumption that theseextend in a x-y plane.

-   Condition 1: The structure must have a continuous network of surface    regions, whose local surface normal vectors n enclose a small angle    with the unit vector z perpendicular to the x-y plane: n≈z (coated    regions).-   Condition 2: The structure between the network from condition 1 must    have as large as possible surface regions whose local surface normal    vectors n enclose a small angle with the direction vector of the    coating b: n≈b (non-coated or shading regions).

Elongate structures are particularly favourable since with these thereresult particularly large surface regions which fulfil condition 2.Furthermore blazed structures are particularly favourable since withtheir steep flank n and b they enclose a particularly small angle if thesteep flank is distant to the coating source.

The surface reliefs A) to C) may be manufactured in a particularlyefficient manner by way of interference lithography. The relief A) maybe manufactured using a positive photoresist. By way of a simplerecopying by way of galvanic or other replication processes, a morefavourable structure B) arises. A similar structure to B) such as e.g.in a hexagonal arrangement may also be manufactured by way ofinterference lithography with three or more incident waves. Elongateholes according to the structure type C) may be very easily manufacturedby way of double exposures with the intermediate rotation of the sampleholder about 1°-85°. With a rotational angle of 1°, the elongation isvery large; with a rotational angle of 85° the elongation is very low.

In the following, the manufacture of microhole structures obtained byoblique exposure, for different application purposes are described.

1. The Manufacture of Filters for Infrared Radiation.

A suitable surface relief is replicated in polyethylene (PE) orpolytetrafluorethylene (PTFE) which are transparent to infraredradiation and is obliquely coated with a metal of a high conductivity,e.g. gold. The filter is capable of functioning after the obliquecoating. The wavelength of the peak transmission is determined by thehole dimensions and the refractive index of the hole, the polarisationdependency on the hole shape. A filter obtained in this manner is shownin FIG. 4 in which the obliquely coated surface relief is imaged suchthat only the metal grid is to be seen. After the oblique coating, aninfrared-transparent protective coating is applied. By way of this, thewavelength of the peak transmission changes.

2. Microhole Structures for the Fine Filtration of Fluids

With the manufacture of microhole structures for the fine filtration offluids, one needs to take several provisions for mechanicallyreinforcing the obliquely coated screen structure. This is effected byway of galvanic reinforcement of the microscreen and by way ofdepositing a support grid. The support grid may either be generateddirectly on the microscreen or separately from this and then depositedonto the microscreen. In the first case the structurisation of thesupport grid may be realised in a particularly economical manner by wayof printing processes. It may be necessary to also galvanicallymanufacture the support grid. In this case the negative structure of thesupport grid is printed. In FIG. 5 an exemplary process course with twogalvanic steps is shown.

In this, a) shows the substrate 3 provided with the surface structure;b) represents this after the oblique coating with a metal 4; and c)represents the arrangement after the galvanic reinforcement of thecoating material with nickel 5. In d) the arrangement with the negativestructure 6 of the support grid consisting of organic material is shownand e) shows the arrangement after a further galvanic treatment withnickel, with which the support grid 7 has been formed. In f) thefinished screen is shown, with which the organic components,specifically the substrate 3 and the negative structure 7 have beenremoved.

It is alternatively possible to print on the support grid itself.

3. The Manufacture of Microhole Structures for Shielding ElectromagneticRadiation.

Microhole structures for shielding undesired electromagnetic radiationare often required on glass surfaces, e.g. cover glasses of plasmadisplays. The essential function of the microhole structure is toachieve a very good direct-current conductivity with a good visualtransmission. The additional task which is accomplished by thisembodiment example is the transfer of a microhole structure onto theglass plate. This is achieved by two variants. In the first variant theglass plate is functionalised on the surface such that the metallicmicrohole structure sticks better to the glass surface than on theobliquely-coated substrate serving as a transfer film. Thisfunctionalisation may also be designed in the form of a vacuum coating,a paint or a sol-gel layer. After the transfer of the microholestructure this may be coated. This variant has the advantage that themicrohole structure is largely resistant with respect to chemical orphysical attacks. The second variant is the lamination of the obliquelycoated substrate on the glass disk. For this application, materials witha large conductivity (e.g. metals) are particularly suitable.

1. A method for manufacturing microhole structures, with which a reliefstructure on the surface of a substrate is obliquely coated with thematerial of the microhole structure, wherein one uses a relief structureof a continuous network of the first surface regions whose local surfacenormal vectors enclose a small angle with the unit vector of thesurface, and of second surface regions which are surrounded by the firstsurface regions and whose surface normal vectors enclose a small anglewith the direction vector of the coating.
 2. A method according to claim1, characterized in that the relief structure of the substrate comprisescylinder-shaped, truncated cone shaped or parallelepiped projections onan at least approximate plane surface.
 3. A method according to claim 1,characterized in that the relief structure of the substrate comprisescylinder-shaped, truncated cone shaped or parallelepiped recesses in atleast approximate plane surface.
 4. A method according to claim 1,characterized in that the relief structure of the substrate is formed byreplication of an original structure.
 5. A method according to claim 1,characterized in that the relief structure of the substrate or theoriginal structure is formed by a photo-lithographic method.
 6. A methodaccording to claim 5, characterized in that the relief structure of thesubstrate or the original structure is formed by interferencelithography.
 7. A method according to claim 6, characterized in that therelief structure of the substrate, or the original structure is formedby an interference lithographic method with a multiple exposure atdifferent angles.
 8. A method according to claim 1, characterised inthat for manufacturing a microhole structure for the radiationfiltration one uses a substrate which is transparent to electromagneticradiation of a certain frequency range.
 9. A method to claim 1,characterized in that for manufacturing a microscreen for the finefiltration, the obliquely coated substrate on the coating side isprovided with a wide-meshed support grid and subsequently the substratematerial is removed.
 10. A method according to claim 9, characterized inthat the wide-meshed support grid is manufactured by way of selectivegalvanic reinforcement.
 11. A method according claim 1, characterized inthat for the manufacture of a microhole structure for the shielding ofelectromagnetic radiation, the material of he microhole structure whichis deposited by way of oblique coating is transferred from the substrateonto a glass surface.
 12. A method according claim 1, characterized inthat for the manufacture of a microhole structure for the shielding ofelectromagnetic radiation, the substrate obliquely coated with thematerial of the microhole structure is lmainated onto a glass surface.13. A method according to claim 1, characterized in that that thematerial of the microhole structure is a metal.
 14. A method accordingto claim 13, characterized in that the material of the microholestructure which is deposited by way of oblique coating is galvanicallyreinforced.